The present invention relates to a system for preventing overheat of an electromagnetic clutch for vehicles, and more particularly to a system for preventing overheat of the electromagnetic clutch as a result of the slippage of the clutch by increasing the clutch current passing through a magnetizing coil of the electromagnetic clutch in proportion of elevation of the temperature of the clutch for decreasing friction in the clutch.
In the electromagnetic clutch, the clutch current flows through a coil to provide an electromagnetic field, so that two members in the transmitting system are connected magnetically with each other to transmit the power from the engine to the transmission. An electromagnetic powder clutch is a form of the above-mentioned electromagnetic clutch.
The electromagnetic powder clutch comprises an annular drive member secured to the crankshaft of an engine, a magnetizing coil provided in the drive member, a driven member installed on the input shaft of the transmission leaving a small gap from the drive member, and a shift lever for changing the gear in the transmission. The shift lever is provided with a switch for the magnetizing coil, which is actuated by operating the shift lever. When the shift lever is shifted to the gear engaging position, the switch is closed making the electric current flow through the magnetizing coil to magnetize the drive member. As the accelerator pedal is depressed, the current applied to the coil increases. The magnetic powder is aggregated in the gap between the drive member and the driven member, bringing about a connection between drive member and driven member. The clutch current passing through the magnetizing coil is progressively increased according to how far the accelerator pedal is depressed, while the clutch slips between the drive member and the driven member.
More particularly, the system is arranged to provide a small clutch current at the start of the vehicle so as to transmit the power in a partial engagement clutch condition, to thereby abate the shock caused by abrupt engagement of the clutch and to prevent the engine-stop. The clutch current is varied with a functional relationship to the engine speed. However, if the electromagnetic clutch slides, keeping the partial engagement condition for a long period, the temperature of the clutch and hence temperature of the coil rise, which causes an increase of the resistance of the coil. In inverse proportion to the increase of coil resistance, clutch current and clutch torque are decreased causing the extension of clutch sliding duration. The longer sliding duration, the higher clutch temperature.